The invention relates to control systems which precisely align electrical components, both as to angular orientation and coordinate (X,Y) location for precise placement via pick-and-place systems such as surface mount component placement machines. More specifically, the invention relates to optically efficient non-contact sensor systems which precisely determine and allow a pick-and-place system to correct the angular orientation of a component and the coordinate positioning of the component for precise placement of the component by a component placement machine on a circuit board or other work surface.
In component placement machines in common use today one or more vacuum quills are used to travel to a bin, pick up a component, properly orient the component and carry the component to a circuit board or other work piece. The component is precisely placed in its proper location with the leads making proper contact with the circuit connections, which are subscribed on the circuit board or work piece. Each electrical component must be placed precisely on the circuit board to ensure proper electrical contact, which requires correct angular orientation and lateral positioning. Angular orientation and lateral positioning may be achieved through mechanical means. A vacuum quill picks up the part to be placed. Four jaws or hammers, suspended from the fixturing device, may travel laterally and strike the component on all four sides with substantially equal force. The intent of such a mechanical system is to shift the component on the vacuum quill so that its angular orientation is correct and so that it is centered on the vacuum quill. The striking of such components can cause damage to the component, such as microcracking of materials commonly used in such components. It is also difficult to achieve the high degree of accuracy, both as to angular orientation and lateral position, that is required by design rules in use in today's technology.
A number of light-based, non-contact methods have been proposed. However, the light-based systems of the past have had difficulty in efficiently achieving both efficiency and the high speed and accuracy required for today's technology. Light sensing systems have also been proposed where a component is interposed in a beam of light and the intensity of the light is detected by a single photodetector or a pair of photodetectors, with a measurement of the maximum light intensity indicating the narrowest shadow, and thus, proper angular orientation of the component. However, it is difficult for such systems to handle the range of components that are placed and to achieve the accuracy required for alignment.
The dimensions of components to be placed today normally vary between components no larger than a grain of sand to component parts in the range of one to two inches. If a single photodetector system is designed large enough to detect shadow variations for a large part, as it must be, the fractional variation caused by rotation of the smallest parts (approximately 0.02 inches) have such little effect on the total light intensity that it is virtually undetectable. For two detector systems, the component part must be precisely aligned between the two detectors with the ratio of light falling on each detector being analyzed to determine edge positions. However, it is extremely difficult to mechanically align photodetectors to make such a measurement.
Finally, it has also been proposed that a series of laser light sources be aligned with a series of laser light detectors. Such a design overcomes some of the problems associated with the proposals for a single detector or pair of detectors. However, calibration of such a system would be difficult and the degree of accuracy that can be achieved can be no more than the spacing of the individual laser sources one from the other. The minimum spacing would be given by the size of a laser diode source, which still would be too large for reliable component position detection. The required physical spacing will also be negatively affected by diffraction effects to further limit the accuracy of such a design. Also, it is believed that the cost of such a system involving many laser sources would also be prohibitively expensive.
Vision based systems using a TV camera are capable of achieving high accuracy. However, they are one of the most expensive of systems proposed and they require a deviation in the path of the quill from the bin to the TV station, and then to the work piece or circuit board, which substantially slows the process. In addition, it is sometimes difficult to distinguish the particular parameters of very small components being placed by such systems from the quill upon which the components are mounted.
A laser sensor system has also been proposed which includes a laser diode, the light from which is collimated with a collimating lens and passed through a slit aperture. This provides a stripe of laser light which passes by and is blocked by the component whose alignment is being sensed. The shadow cast by the component is detected by a linear array detector. Data read from the detector array is analyzed to detect the leading edge and the trailing edge of the shadow which is cast upon the detector array. Since only the shadow edges are detected and analyzed, the same degree of accuracy is achieved when aligning a 0.02 inch part as is achieved when aligning a 2.0 inch part. However, this highly accurate system utilizes only a small portion of the laser energy generated and is sensitive to optical defects, most particularly in those small portions of the lens through which light is passed to the sensor array. Therefore, what is needed is a sensor system which captures substantially more of the emitted energy for the purposes of differentiating between shadow and light. What is needed is a sensor system for creating sharper component shadow images on the detector using any of several different light sources including low powered laser and LED light sources.